In the typical delayed coking process, high boiling petroleum residues are fed to a large upright vessel, called a coke drum, where they are thermally cracked into light products and a solid residue--petroleum coke. When the drum is full, the feed is diverted to an empty drum and the coke-filled drum is steam purged and cooled with quench water. The drum is then drained of water, and the top and bottom heads are removed to allow the coke to be cut from the drum by high pressure water jets. After the coke is removed, the heads are replaced and the coke drum is readied to be placed back into service to repeat the cycle.
The delayed coker unit has generally been operated at a lower than optimum efficiency to ensure the formation of sponge coke because sponge coke accumulates on the walls of the drum where it is supported when the bottom head is removed. To maximize performance, refinery operators are increasingly using lower grade feedstocks which increase the prevalence of shot coke production. With a pellet-like consistency, shot coke is not supported by the walls of the coke drum. As a result, shot coke produces a large load on the bottom head and makes the head more difficult to remove. In addition, the coke can fall out of the vessel as the bottom head is opened and pose a hazard to personnel and equipment in the immediate vicinity.
To minimize this hazard, numerous attempts have been made to make the head removing operation in a decoking process increasingly automated to reduce the presence of maintenance personnel and equipment near the drum. Nagy et al., Oil & Gas Journal, pp. 77-80, May 29, 1989 and EP Application No. 89-307,539, for example, describe a head operating mechanism which is said to improve delayed coker safety and efficiency. After the bolts are removed manually, hydraulic cylinders can be remotely activated to swing the head open on a hinge to allow access for coke removal. However, a hinged head cannot be withdrawn for maintenance work, i.e. cleaning and gasket replacement, until the decoking operation has been completed. Personnel must service the head under the opened drum from which debris can fall. In addition, extensive vessel fabrication work must be undertaken to retrofit existing coker drums, i.e. the hinge must be installed on the bottom flange.
U.S. Pat. Nos. 4,726,109 to Malsbury et al. and 4,960,358 to DiGiacomo et al. describe a remote unheading device for coking drums. The device includes a head unit for attachment to a lower flange of a coking drum and a plurality of swing bolts which are disconnected by remotely operated detensioning equipment. A platform device lowers the head unit, moves it laterally to one side and tips it for cleaning. A chute attached to the frame can be raised into engagement with the coking drum lower flange for removal of coke from the drum. Although remotely operable, shot coke presents a problem since the chute cannot be engaged until the head has been completely withdrawn. Installation of this device on existing cokers also requires extensive vessel fabrication work.
Hahn & Clay, FACT Closure Installation, Operation, and Maintenance Manual, May 1991, describes a closure device for coke drum unheading operations. A conventional bottom head is replaced with a custom head drilled for thirty-two bolts. The head is assembled as a sandwich with another flange or force ring of the same size as the bottom head. The head and force ring are assembled with a series of ramps located along the perimeter of the head. The assembly is held in place by the bolts and high nitrogen pressure is applied to a special force actuator located between the flanges to move them apart. Then a set of three hydraulic cylinders turn a special ramp ring to push the head upwardly from the force ring flange to hold the head in a pressurized position after the nitrogen pressure is removed. While suitable for remote operation, this device also cannot effectively deal with shot coke, is relatively complicated and cannot be easily retrofit on existing equipment.
U.S. Pat. Nos. 5,228,825 to Fruchtbaum et al. and 5,294,157 to Smith et al. disclose a coke drum deheading device which has springs and hooks with a retaining element to automate the head removal/replacement process. While a significant improvement over the other deheading devices, these disclosures do not address the handling of shot coke.
Consequently, there is a need for a delayed coker unheading device which can facilitate the performance of necessary service without exposing personnel to the danger of falling coke, and provide for capture of falling coke or other debris to maintain operability in the event of shot-coke cave-in. Further, the device should be relatively easily retrofitted on existing equipment and simple to operate and maintain.